The subject matter described herein relates generally to controlling operation of electric power converters, and more specifically, to controlling operation of insulated gate bipolar transistors (IGBTs).
Many known semiconductor devices are used for electric power conversion, e.g., rectifiers and inverters. Most known rectifiers are used for converting alternating current (AC) to direct current (DC) and most known inverters are used for converting DC current to AC current. Some of these rectifiers and inverters are integrated into full power conversion assemblies, i.e., power converters, used in renewable electric power generation facilities that include solar power generation farms and wind turbine farms. However, variables such as solar intensity and wind direction and speed typically produce electric power having varying voltage and/or frequency. Power converters may be coupled between the electric power generation devices in the generation facilities and an electric utility grid. Each power converter receives generated electric power from the associated generation device and transmits electricity having a fixed voltage and frequency for further transmission to the utility grid via a transformer. The transformer may be coupled to a plurality of power converters associated with the electric power generation facility.
Many known semiconductor devices include power bridge circuits that include insulated gate bipolar transistors (IGBTs). Each IGBT includes a gate coupled to a low voltage control circuit, wherein such low voltage is typically defined on the order of magnitude of volts (V). Each IGBT also includes a collector and an emitter coupled to a high voltage circuit, wherein such high voltage is typically defined on the order of magnitude of kilovolts (kV). A load is typically coupled to the collector. During operation, when the IGBT is in an “on-condition”, energy is stored in the load and in a channel that extends between the collector and the load and a channel that extends between the collector and the emitter. The stored energy is proportional to the sum of the inductances associated with the load and the channels, such inductances typically referred to as stray inductances. During IGBT turn-off transients, a voltage excursion is induced between the collector and the emitter of the associated IGBT, such voltage typically referred to as VCE. The VCE increase is proportional to a rate of change of electric current as a function of time, i.e., a di/dt, and the sum of the stray inductances. The value of the di/dt may be on the order of thousands of amperes per microsecond. Such VCE excursions may decrease the life expectancy of the IGBT.
At least some known IGBTs include “active clamping” circuits that include a plurality of Zener diodes coupled in series between the gate of the IGBT and the collector on the low voltage side of the IGBT. The series Zener diode circuit may also include resistors, clamping diodes, and switches to facilitate clamping the voltage at the collector to a predetermined value that is typically the sum of the voltage drop across each Zener diode. Such voltage clamping is facilitated by the Zener diodes permitting electric current to flow in the reverse direction when the collector voltage exceeds the predetermined breakdown voltage, sometimes referred to as the “Zener knee voltage.” The voltage clamping is further facilitated by maintaining the IGBT in a partial “on-state” for a short period of time, thereby facilitating some current to circulate through a circuit defined by the gate and the collector of the IGBT and the Zener diodes. The energy associated with the voltage transient dissipates as heat energy as the induced electric current is transmitted through the circuit, thereby limiting the voltage excursion. However, the increased temperatures due to the heat release may decrease the service life of the IGBT and the Zener diodes.
Moreover, such known clamping circuits must be uniquely designed and assembled for each IGBT configuration and operational use. Specifically, the clamping threshold is difficult to adjust for each individual direct current (DC) bus voltage rating as there is no easy way to tune the knee voltage of the Zener diodes. Therefore, each individual power bridge circuit typically requires substantial experimentation and testing, and assembling a clamping circuit for variable voltages is not practical. Also, for those power bridge circuits having consistent DC bus voltage ratings, variations of the stray inductances associated with the IGBT and the load will induce voltage variations at the collector that may not be easy to control. Further, a failure of one Zener diode in the series circuit may induce a “false turn-on” for an IGBT in an otherwise “off-state”, thereby potentially shortening the life expectancy of the associated IGBT. Moreover, if the series Zener diode clamping circuit is repeatedly activated, the thermal limits of the Zener diodes may be approached, thereby limiting the effectiveness of the affected Zener diodes and reducing the effectiveness of the associated clamping circuit. Known methods to better protect the Zener diodes in the clamping circuit include increasing thermal capacities of the hardware, adding a switch to disconnect the Zener clamping circuit, and draining excessive gate charges to facilitate softer, i.e., extended IGBT turn-off. However, the root cause of the potential failure conditions of the Zener diodes, i.e., a lack of active clamping circuit regulation of the voltage excursions, is not addressed by these known methods.